1. Field of the Invention
The present invention relates to a process for forming a refractory repair mass, in particular to a process for forming a refractory repair mass on an alumina-containing surface. It is especially concerned with the repair of an alumina-containing material which is to be exposed to heavy duties, for example to contact with molten aluminium or to the severe conditions encountered in a glass tank at the "glass line" (the upper surface of the molten glass).
2. Description of the Related Art
The process uses a technique of the type generally known as "ceramic welding", in which a mixture of solid refractory particles and solid combustible fuel particles of a material which generates a refractory oxide are projected against the surface to be repaired and the fuel is there reacted with oxygen-rich gas, usually substantially pure oxygen, such that the heat of reaction is released against the surface so that a coherent refractory repair mass is formed.
Such "ceramic welding" is described in GB patent 1,330,894 (Glaverbel) and GB 2,170,191 (Glaverbel). The combustible particles are particles whose composition and granulometry are such that they react in a strongly exothermic manner with the oxygen to form a refractory oxide while releasing the necessary heat for melting, at least superficially, the projected refractory particles. The projection of particles is conveniently and safely achieved by using the oxygen as a carrier gas for the particle mixture. In this manner a coherent refractory repair mass is formed against the surface on to which the particles are projected.
These known ceramic welding processes can be employed for forming a refractory article, for example, a block having a particular shape, but they are most widely used for forming coatings or for repairing bricks or walls and are particularly useful for repairing or reinforcing existing refractory structures.
Alumina-based refractory materials display good resistance to thermal shock and for this reason are widely chosen for the refractory blocks used for severe duties in the steel, non-ferrous (aluminium and copper) and glass industries. For example, blocks of AZS (alumina together with silica and zirconia) are used at the liquid level in a glass tank furnace. Electrofused "Zac" (trade mark) bricks contain for instance 50-51% by weight alumina, 15-16% silica and 32-33% zirconia. Higher alumina contents are present in the blocks used in constructing aluminium smelting/melting furnaces, e.g. material containing 60 to 85 wt % alumina and 5 to 35 wt % silica together with small amounts of a cement.
Ceramic welding is well suited to the repair of alumina-containing refractories such as AZS and higher alumina containing material. These refractories are exposed to service temperatures up to 1100.degree. C. in the aluminium industry and even higher in gass furnaces. As with most other types of furnace, it is desirable that repairs are conducted while the furnace remains hot, e.g. keeping a wall to be repaired at a temperature of at least 500.degree. C., desirably at least 800.degree. C.
In some cases, the repair mass must resist erosion and corrosion by molten material, e.g. molten aluminium in the aluminium industry, and must display good compatibility with, and adhesion to, the surface to be repaired. In the case of aluminium smelting/melting furnaces the refractories are affected by the molten material, which may contain magnesium in addition to aluminium. Both these molten metals react with the refractory such that with the passage of time the crystalline structure at the surface and increasingly deeply into the interior of the material progressively includes corundum (Al.sub.2 O.sub.3) and spinel (MgO.Al.sub.2 O.sub.3). The thermal expansion of the surface is correspondingly modified, becoming substantially higher than that of the virgin material. It is thus necessary to apply a repair mass which is compatible with the modified material and resistant to molten metal.
For AZS refractories used in glass furnaces one means of protecting their surface against erosion or corrosion is to apply a coating of a refractory metal such as platinum. In this case it is necessary to provide a dense, non-porous, surface before depositing the metal on it. A surface of this quality is obtained by coating the base refractory with a refractory layer formed by ceramic welding.